This invention relates to the analysis of gases containing molecular fluorine, and more particularly to a method for determining at least the concentration of fluorine in a fluorine containing mixed gas and an apparatus for same. The method and apparatus according to the invention are particularly suitable for analyzing excimer laser gases containing fluorine.
Known methods for determining the concentration of fluorine in a mixed gas include wet methods and dry methods.
A typical example of the wet methods is an electroanalysis method using a LiCl solution cell in which chlorine ion is oxidized by absorption of fluorine gas. (Anal. Chem., 40, 2217 (1968).) In this method it is important to accomplish complete absorption of the sample gas in the LiCl solution, but in industrial applications it is troublesome to maintain optimum conditions of blowing the gas into the solution, and the analysis entails intricate instruments and operations.
An example of the dry methods has the steps of passing a fluorine containing gas through a column of NaCl to form chlorine gas, forcing the chlorine gas to be absorbed in NaOH solution and determining the hypochlorite formed in the solution by iodometric titration. (A.M.G. Macdonald et al, "Fluorine", Encyclopedia of Industrial Chemical Analysis, Vol. 13, New York-London-Sydney-Tronto, 1971.) Since this method uses two sequential reactions the instruments become complicated, and it is difficult to enhance accuracy. Also it is known to directly analyze a fluorine containing gas by gas chromatography with corrosion preventing measures such as use of nickel or an alternative special material for the parts contacting the sample gas and covering the thermal conductivity cell with PTFE. However, such measures raise the cost of the apparatus and reduce the sensitivity.
Meanwhile, some excimer lasers are using a fluorine containing gas such as a mixture of Ar, F.sub.2 and He or Ne or a mixture of Kr, F.sub.2 and He or Ne. In such a laser gas the content of F.sub.2 is usually about 0.1-1 vol %. It is not seldom that during operation of an excimer laser of this category the laser output power gradually lowers mainly by reason of lowering of the concentration of fluorine in the laser gas and/or formation of impurity gases. Although impurity gases formed in an excimer laser gas can be removed periodically or continuously by an adsorption process or a cold-trap process, monitoring of fluorine concentration in the laser gas is presently almost impracticable because of intricateness of analyzing operations and difficulty of handling the sample gas.